1. Field of the Invention
The present invention relates to a bar code system for Brownie film and a bar code reader of a camera for reading bar code from the Brownie film.
2. Background Arts
ISO 120 type and 220 type rolled films are called Brownie film roll in the art. In the 120 type, a 6 cm wide photo filmstrip is secured by an adhesive tape to a light-shielding paper which is longer than the filmstrip and backs the entire length of the filmstrip. The filmstrip is coiled with the light-shielding paper around a spool. In the 220 type, two sheets of light-shielding paper are spliced by adhesive tapes to leading and trailing ends of a 6 cm wide photo filmstrip, and are coiled together around a spool. The light-shielding paper shields the filmstrip from ambient light before and after use of the film roll. Besides, the 120 type is graded into two sizes according to the length of the filmstrip. Both the 120 type and the 220 type are classified by the film speed, divided into negative and reversal, and categorized as color film roll or as black-and-white film roll.
In conventional medium-sized cameras, or called Brownie cameras, the pressure plate and the frame counter are manually adjusted to the film roll type and the film length, and the film speed is manually entered each time a new film roll is loaded. To simplify or automatize these setting up operations, U.S.P. Application Ser. No. 08/712,387 suggests providing a bar code on the adhesive tape at the leading portion of the Brownie film. By providing the medium-sized camera with a bar code reader using a reflective photo sensor, it is possible to read the bar code during first frame setting, that is, while the Brownie film is initially advanced to position a first frame exposure area behind an exposure aperture.
Bar code in general consists of alternating high reflective and low reflective bar elements, hereinafter called white and black bar elements respectively. The bar elements have either a narrow width or a wide width regardless of whether it is black or white. The wide bar elements represent binary "1", whereas the narrow bar elements represent binary "0". Since the bar elements should extend orthogonally to the advancing direction of the Brownie film to read it during the film advancing, the width of each bar element means the length in the film advancing direction, i.e. the lengthwise direction of the filmstrip. According to the ISO standard, the length of the adhesive tape of the Brownie film roll in the lengthwise direction of the filmstrip is limited up to 25 mm. Therefore, when the bar code is to be provided on the adhesive tape, the entire length of the bar code in the lengthwise direction of the filmstrip is limited correspondingly. The number of bar elements and the narrow and wide widths of the bar elements are limited by the entire length of the bar code as well as the requisite number of bits. For reliable discrimination between the wide bar element and the narrow bar element, the wide bar width should preferably be about 2.5 times the narrow bar width. It is also necessary to provide quiet zones before and behind the bar code elements. To provide a 13-bit bar code, the narrow bar width is about 0.8 mm.
To distinguish the wide bar element from the narrow bar element, borders between the white bar elements and the black bar elements, i.e. edges of the black bar elements, are detected, and the width of each bar element is measured as a spacing between two adjacent borders. If the reflection densities of the white bar elements and the black bar elements are respectively constant, and differ sufficiently from each other, the borders between the black and white bar elements can be detected by the reflective photo sensor with high accuracy.
On the other hand, since the reflectance of the adhesive tape is generally higher than that of the light-shielding paper, it is possible and efficient to print black bar elements directly on the adhesive tape, while using the obverse surface of the adhesive tape as white bar elements. In that case, however, since the reflectance of the adhesive tape varies depending upon the film manufacturer, amplitude of output signal from the photo sensor also varies even if the black bar elements have an uniform reflectance. For this reason, it is practically difficult to reliably discriminate between the black and white bar elements by use of a constant threshold value in any Brownie film rolls.
Moreover, since the adhesive tape as well as the filmstrip curls inevitably while it is rolled around the spool, the distance from the obverse surface of the adhesive tape to the photo sensor varies up to 1 mm or so, as the adhesive tape passes in front of the photo sensor. The variation in the distance results a variation in the sensor output signal. Therefore, using a constant threshold value for the border discrimination tends to cause reading failure, even if the reflectance of the adhesive tape and that of the black bar elements are respectively standardized for any Brownie film rolls. Because the photo sensor of the camera can scan the bar code only once during the first frame setting of the Brownie film, it is necessary to check if the sensor output signal is free from errors, in order to decode the bar code with accuracy based on the sensor output signal.
After the borders between the bar elements are detected, and the width of each bar element is measured, the binary value of each bar element is determined by comparing the measured width to a reference width. Using a fixed value as the reference width leads to restricting the frequency of clock pulses that are used for measuring the width of each bar element, and thus restricting the design of the bar code reader. Tolerance in width of the respective bar elements is also restricted, so that the manufacturing cost is raised.
It is possible to provide a preliminary bar code before the bar code in the film advancing direction, such that the preliminary bar code have a constant bit pattern so the camera can determine a reference width based on the data detected from the preliminary bar code. Thereby, the reference width is adjusted automatically in each camera for each Brownie film roll. However, since the length of the adhesive tape in the film advancing direction is limited, it is necessary for adding the preliminary bar code to reduce the widths of the respective bar elements or the number of bar elements of the bar codes. To record sufficient amount of data, the number of bar elements, i.e. the number of bits, cannot be reduced so much. To detect the bar elements with reduced widths, an expensive bar code reader with high resolving power is necessary.